The present invention relates generally to the field of medical devices for implantation into humans. More particularly, it concerns methods for processing biological tissues for use as bioprosthetic devices.
Bioprostheses are devices derived from processed biological tissues to be used for implantation into humans. The development of such devices originated as an attempt to circumvent some of the clinical complications associated with the early development of the mechanical heart valve, and has since resulted in a rapid proliferation of bioprosthetic devices for a variety of applications. Examples of some of the bioprostheses currently used or under development include heart valves, vascular grafts, biohybrid vascular grafts, ligament substitutes, and pericardial patches.
The primary component of the biological tissues used to fabricate bioprostheses is collagen, a generic term for a family of related extracellular proteins. Collagen molecules consists of three chains of poly(amino acids) arranged in a trihelical configuration ending in non-helical carboxyl and amino termini. These collagen molecules assemble to form microfibrils, which in turn assemble into fibrils, resulting in collagen fibers. The amino acids which make up the collagen molecules contain side groups, including amine (NH2), acid (COOH) and hydroxyl (OH) groups, in addition to the amide bonds of the polymer backbone, all of which are sites for potential chemical reaction on these molecules.
Because collagenous tissues degrade very rapidly upon implantation, it is necessary to stabilize the tissue if it is to be used clinically. Chemical stabilization by tissue cross-linking, also referred to as tissue fixation, has been achieved using bi- and multi-functional molecules having reactive groups capable of forming irreversible and stable intra- and intermolecular chemical bond formation with the reactive amino acid side groups present on the collagen molecules.
Glutaradehyde is a bifunctional molecule capable of reacting under physiological conditions with primary amine groups on collagen molecules. Although it is the most commonly used chemical fixative for biological tissues, there are a number of drawbacks associated with its use in the production of bioprosthetic devices. For example, the long term durability of glutaraldehyde-fixed bioprostheses is not well established, particularly in view of a number of reports of mechanical failures of the tissue at points of high mechanical stress (Broom, 1977; Magilligan, 1988). Another drawback to glutaraldehyde fixation of bioprostheses relates to the depolymerization of the cross-links that has been observed in vivo, resulting in release of toxic glutaraldehyde into the recipient (Moczar et al., 1994; Wiebe et al., 1988; Gendler et al., 1984).
In addition, glutaraldehyde-fixed biprostheses have an undesirable propensity to calcify after implantation. This calcification appears to represent the predominant cause of failure of glutaraldehyde-fixed devices (Golomb et al, 1987; Levy et al., 1986; Thubrikar et al., 1983; Girardot et al., 1995). Increased calcium uptake within a bioprosthesis leads to an accumulation of calcium phosphate, which in turn mineralizes into calcium hydroxyapatite. The calcification process is not well understood, but appears to depend on factors such as calcium metabolism diseases, age, diet, degeneration of tissue components such as collagen, and turbulence. Calcification of bioprostheses has been associated with degenerative changes in glutaraldehyde-treated collagen fibers.
A number of approaches have been investigated for reducing calcification of glutaraldehyde-fixed bioprostheses. For example, glutaraldehyde-fixed bioprosthetic heart valves have been treated with surfactants to reduce the calcification following implantation (U.S. Pat. No. 5,215,541). In other examples, alpha-aminooleic acid treatment of glutaraldehyde-fixed tissue has been reported as an effective biocompatible, non-thrombogenic approach for minimizing calcification of bioprosthetic devices (Girardot et al., 1991; Gott et al., 1992; Girardot et al., 1993; Hall et al., 1993, Myers et al., 1993; Girardot et al., 1994). The broad applicability of this approach in the production of bioprostheses, however, may be limited by the inability to achieve good tissue penetration of the alpha-aminooleic acid into the already glutaraldehyde-fixed tissue (Girardot, 1994).
With respect to the biocompatibility of prosthetic devices, application of most non-physiological biomaterials and prostheses to living tissues initiates a series of physiological events which can activate host defense mechanisms such as coagulation, platelet adhesion and aggregation, white cell adhesion, complement activation, etc. In attempts to improve the biocompatibility/hemocompatibility of articles adapted for use in contact with blood or blood products, aliphatic extensions have been added to the surface of polymeric biomaterials in order to provide hydrophobic binding sites for albumin. The binding of albumin to the prosthesis has been reported to provide a low activation of coagulation, low complement activation, and reduced platelet and white cell adhesion, thereby providing improved hemocompatibility (See for example U.S. Pat. No. 5,098,960; U.S. Pat. No. 5,263,992; Munro et al., 1981; Eberhart, 1989).
Some cross-linking agents have been investigated as alternatives to glutaraldehyde. These have been based on compounds such as polyepoxides, diisocyanates, di and polycarboxylic acids, photooxidation using organic dyes, etc. (see Khor, 1997, for review).
Thus, there is a need within the field of bioprosthetics for simple, cost-effective methods for cross-linking biological tissues which overcome some of the limitations associated with glutaraldehyde and other fixation approaches used in the art and which provide bioprosthetic devices with desirable mechanical characteristics and biocompatibility and a reduced susceptibility to calcification relative to glutaraldehyde-fixed tissues.
This invention broadly concerns methods for cross-linking biological tissue, and the cross-linked tissue so produced, comprising treating the tissue under effective cross-linking conditions with a compound which has an aliphatic component, typically containing from about 8 to about 40 carbon atoms, in addition to at least two chemical constituents/functionalities that are reactive with collagen. The collagen reactive groups can be essentially any chemical functionalities that are reactive with one or more of the amino acid side chains of collagen or with the collagen amide backbone. Preferred collagen reactive groups include isocyanate, epoxy, and n-hydroxysuccinimide. The aliphatic component of the cross-linking agent can be any linear or branched, saturated or unsaturated, aliphatic chain.
The disclosed invention provides an alternative approach to glutaraldehyde and other cross-linking agents for treating biological tissues, particularly those intended for use as bioprosthetic devices, and offers a number of advantages including good mechanical characteristics, biocompatibility, and a reduced susceptibility to calcification of the cross-linked tissue after implantation.